This invention relates generally to electrostatic transfer media for forming permanent print copies of images formed by electrostatic methods and more particularly, provides an opaque image receptor capable of receiving a toner image from an imaged electrophotographic carrier forming an opaque reflective print copy thereof, the toner image being contact transferred to the image receptor and embedded therein by application of heat and pressure, the resulting reflective print copy having substantially increased depth intensity and contrast over use of available silver halide photographic film.
Formation of print copies electrostatically by transfer of toned electrostatic images from an electrophotographic member to a secondary carrier is advantageous in the qualities of speed, in that it employs generally chemically-free procedures, does not require skilled technicians and does not require darkroom or other special conditions and equipment. However, where a photographic quality is sought, the advantages of electrostatic reproduction are offset by the resulting generally poor resolution and the considerable loss in optical density. Edge effect often characteristic of most electrostatic reproduction procedures has reduced the acceptance of electrostatic processes in high resolution print making.
Various processes have been proposed for producing an image upon a substrate, including photographic processes involving actinic exposure of a photosensitive material carried on a substrate or electrostatic process involving exposing a charged electrophotographic member having a photoconductive surface coating or layer to radiation to produce an electrostatic latent image. This latent image is rendered visible by application of dry toner particles thereto as in cascade type development, or by wet application thereto of a liquid toner suspension wherein the toner particles have electrophoretic properties.
The production of suitable print reproductions heretofore commonly requires the skill of a trained technician and the substantial expenditure of money and time. Photographic reproduction processes require controlled exposure, development, washing and fixing of a light sensitive composition present on a support with or without the intermediate production of a negative image.
Photographic reproductions generally are formed by chemically reacting a photosensitive emulsion layer or layers bonded to a substrate or base carrier. Each layer is reacted with the depth of the incremental reactions extending fully through the layer of each layer, where plural layers are involved. With the full thickness of the layer at any one portion of the layer comprising the reacted medium, the image cannot be backlighted or have any backlighted effect. Accordingly, the depth of the resulting image is limited.
Xerographic processes have proven to be an easy and reliable technique for the production of reproductions. Notwithstanding the desirability of these imaging processes, drawbacks have been encountered in forming print reproductions in that the adherence of the image on the transfer support leaves much to be desired. Additionally, some loss of optical density and resolution is experienced upon transfer of the toned image to a receiving member employing prior methods.
Electrophotographic processes require the provision of a suitable image carrier upon which images are formed, these carriers being required to accept an electrical charge and retain the charge sufficiently to enable an image to be formed by application of toner particles thereto. Many materials displaying photoconductivity will not accept a charge initially, and of those which may be charged, few are capable of retaining the charge thereon without leaking off or decaying so rapidly as to be almost useless. In addition to accepting a charge and retaining the charge in darkness, the photoconductive layer is required to discharge in light areas to a degree which is fairly rapid and generally proportional to the amount of light to which the surface is exposed impinging upon the charged surface. Further, there must be retained a discernible difference between the remaining charged and uncharged layers without lateral movement of the charges.
In U.S. Pat. No. 4,025,339, an electrophotographic member is provided with an outer coating of a unique photoconductive material comprising a uniform, microcrystalline, highly ordered, wholly inorganic sputtered deposit having unusual electrical and optical properties particularly advantageous in electrostatic reproduction processes. The patented photoconductive coating is electrically anisotropic, electric anisotropy effectively resulting from the field domain of each crystal forming the coating functioning independently in the charge mode and in the discharge mode without lateral translation to contiguous crystals. Optical anisotropy is believed to be a result of the single crystal activity of the coating.
The uniform vertical orientation of all crystallites defining the photoconductive coating is believed to be a key factor in both the electrical and optical anisotropy demonstrated by the said coating. During toning in the course of the electrostatic reproduction process the toner particles are attracted by myriads of individual fields each having different magnitude individual field strengths, enabling resolution to be achieved which heretofore was unobtainable by then conventional electrostatic reproduction processes.
An electrostatic transfer medium was described in pending application Ser. No. 317,445 filed Nov. 2, 1981 owned by Assignee herein, comprising a transparent polyester sheet having a thin layer of thermoplastic compatible resinous material bonded thereto. The thermoplastic layer had a softening temperature range sufficient to enable toner particles from a toner image on an image carrier to pass thereinto when the layer was heat softened and become embedded. The transfer process there disclosed involved (1) heating the transfer medium sufficient at least to soften the thermoplastic layer and (2) the engagement of the toned, electrostatic latent image-carrier image side down upon the softened coating while pressure, as from a roller, is applied to form a sandwich laminate. The resulting laminate is cooled and then separated into its two component sheets with the result that the entire toned image is transferred intact leaving no residue upon the carrier. The disclosed process was limited to making of transparencies, preferably employing flexible transparent, thin polyester sheets having an affinity for polyester family resins. Full transfer with minimal loss in resolution yet with retention of optical density values was achieved. The substrates were generally smooth, uniformly surfaced sheets on which the thermoplastic polymer was applied. Transfer to less smooth surfaces would greatly enhance the commercial utility of such type processes.
Using conventional electrophotographic processes, it was not possible to form or to transfer toner iamges to surfaces of roughened or irregular surface configuration or to surfaces of stretchable media such as inflatables. It would be of considerable utility to provide a method whereby the transfer of electrostatically obtained toner images could be applied to surfaces independent of their surface configuration and thus provide improvement over the silk screen type processes conventionally employed for such materials.
Electrostatic print reproductions generally have compared unfavorably to photographically obtained print copies in that the former lack the depth, contrast, resolution perceived from the latter type prints. It would be highly desirable to provide print copies using electrophotographic methods but which are even superior to the conventional photographic prints, which have high resolution, improved contrast, depth and intensity, which have a three dimensional effect upon viewing when compared to the conventional photographic print.
Further, considerable product and process advantage would ensue, if in addition to an improved brilliance of image, a process could be provided where one would start with a high resolution, inorganic, reusable photoconductor which would be first toned and whose release properties permit pressure and heat to be applied during contact transfer of the toned image facilitating the full encapsulation of the toner image without detectable lateral image spread or change in density and resolving power on the print as a result of the transfer operation.